1. Field of the invention
The present invention relates to a CMOS crystal controlled oscillator.
2. Description of the Prior Art
A CMOS crystal controlled oscillator is essential to microprocessors and microcomputers as a clock source thereof.
A CMOS oscillator comprises a CMOS inverter, a crystal element, a capacitor and a feedback resistor, and is operatable with low power consumption. One such oscillator is disclosed in U.S. Pat. No. 3,676,801 in detail.
To further decrease the power consumption of the CMOS crystal control oscillator, resistors R1 and R2 are additionally added thereto for limiting an operation current of a CMOS inverter, as shown in FIG. 2.
In a CMOS oscillator of this type, the CMOS inverter and the feedback resistor are formed on a semiconductor substrate as a chip, and a crystal element is connected with a terminal of an IC package as an external part.
A circuit constant of the CMOS oscillator in the IC chip is usually designed such that the oscillator is operated at the maximum frequency of oscillation with the minimum power as shown in (A) of FIG. 3. Accordingly, provided that the oscillator is needed to generate a low frequency clock signal with use of a low frequency crystal element, it is feared that it oscillates at a doubled frequency of the clock signal. In addition, since the voltage amplitude of the clock signal is increased as shown in waveform (B) of FIG. 3, the oscillator requires a higher power consumption.
Particularly, upon operating a high-speed microprocessor or microcomputer with use of a low frequency oscillation signal as a clock, a current-limiting resistance does not satisfactorily exert its effect and causes the power consumption of the oscillator to be increased. Furthermore, improper selection of a current-limiting resistance value may cause the osillator to oscillate at a doubled frequency.